The present invention relates to an information transmission system employing optical communication, and more particularly to a network with high reliability and flexibility using optical frequency selection and optical frequency conversion functions.
Recently, with the advance of coherent communication techniques, there has been proposed a network utilizing optical frequency division multiplexing (or optical wavelength division multiplexing) transmission.
Typical examples of the optical frequency or wavelength division multiplexing network are found in paper (1) "IEEE Journal of Lightwave Technology, Vol. 7, No. 11, pp. 1759-1768, 1989" and paper (2) "Proceedings of IOOC, '90, pp. 84-95, 1990". Networks described in other papers are similar to those described in the above two papers.
A network configuration described in the paper (1) is shown in FIG. 2 of the paper and part thereof corresponding to the present invention is shown in FIG. 2 of the accompanying drawings. FIG. 2 shows a line distribution and collection system of the network shown in the paper (1). The system of FIG. 2 includes a remote node 10 having a wavelength demultiplexer 500 and a wavelength multiplexer 501 connected through optical fibers 100 and 200, respectively, to a central office and subscriber terminals 20-1.about.N connected through optical fibers 300-1.about.N to 400-1.about.N to the remote node. Signals having wavelength .lambda..sub.1l to .lambda..sub.1n transmitted from the central office in wavelength division multiplexing fashion are demultiplexed into signals having the respective optical frequencies by the wavelength demultiplexer to be transmitted to the subscriber terminals 20-1.about.N. On the contrary, signals having wavelength .lambda..sub.2l to .lambda..sub.2n transmitted from the subscriber terminals 20-1.about.N are wavelength-multiplexed by the wavelength multiplexer to be transmitted to the central office.
In the above-mentioned system, the subscriber terminals 20-1.about.N must transmit and receive signals having different wavelengths, respectively. In the paper (1), as shown in FIG. 4 thereof, receivers are common to the subscriber terminals, while transmitters employ lasers having different wavelengths for each subscriber terminal. Accordingly, a laser having stable wavelength must be provided in each subscriber terminal and hence there is a problem in reliability and flexibility. Further, movement of the subscriber terminal is not easy.
In the paper (1), transmission employs the conventional intensity modulation optical communication and accordingly it is difficult that the multiplex degree of optical signal exceeds 100. Even in this system, a coherent receiver capable of effecting multiplexing with the multiplex degree of 1000 or more can be used. In this case, receivers capable of receiving signals having wavelengths .lambda..sub.1l to .lambda..sub.1n transmitted from the central office assigned to the subscriber terminals 20-1.about.N with wavelength division multiplexing are required. Accordingly, the receivers are expensive as compared with the present invention described later.
Further, coherent receivers having variable transmission wavelength and common to the subscriber terminals 20-1.about.N can be employed. In this case, however, signals having wavelength .lambda..sub.2l to .lambda..sub.2n transmitted from the subscriber terminals are also multiplexed and accordingly the wavelength must be stable. It is difficult to remotely control the wavelength and hence the reliability of the network is also degraded.
Furthermore, when it is to be attempted that the optical fibers 300-1.about.N and 400-1.about.N are combined to effect bi-directional transmission by means of a single optical fiber per subscriber terminal, "it is basically required that all of wavelengths .lambda..sub.1l to .lambda..sub.1n and .lambda..sub.2l to .lambda..sub.2n are different" and utilization efficiency of frequency is deteriorated.
A network configuration described in the paper (2) is shown in FIG. 1 of the paper and is shown in FIG. 3 of the accompanying drawings in corresponding manner to the present invention. The system includes a remote node (not shown in the paper (2)) having a power divider 502 and a transport star coupler or wavelength multiplexer 501 connected to a central office (not shown in the paper (2)) through optical fibers 100 and 200 and fixed wavelength receivers and tunable transmitters or subscriber terminals 20-1.about.N connected to the remote node through optical fibers 300-1.about.N and 400-1.about.N. All optical signals having wavelengths .lambda..sub.1l to .lambda..sub.1n transmitted from the central office with wavelength division multiplexing are transmitted to the subscriber terminals 20-1.about.N by means of the power divider and the subscriber terminals 20-1.about.N receive only necessary signals by receivers for receiving only particular wavelength. On the contrary, signals having wavelengths .lambda..sub.2l to .lambda..sub.2n transmitted from the subscriber terminals are wavelength-multiplexed by the wavelength multiplexer to be transmitted to the central office.
This system is featured in that an inexpensive power divider is used instead of the wavelength demultiplexer of the paper (1) and wavelength selection reception which is a maximum advantage of coherent transmission can be utilized.
The maximum drawback of this system is that all of the subscriber terminals 20-1.about.N can receive all signals. Thus, there is a problem in privacy characteristic.
Accordingly, in the system of the paper (2), receivers having a fixed receive frequency are disposed in each of the subscriber terminals 20-1.about.N. However, there remains the problem in the privacy characteristic for malicious operation.
Further, when a coherent transmitter and receiver are used, the transmitter and receiver of the system have also the same problem as in the transmitter and receiver of the paper (1).
The conventional network utilizing the wavelength division multiplexing has drawbacks as follows. Particularly, since the wavelength employed between the central office and the remote node and between the remote node and the subscriber terminals is the same, a failure occurring in one subscriber terminal influences all of the subscriber terminals connected to the remote node to which the subscriber terminal having the failure is connected. Further, since the transmitter and receiver of the subscriber terminal must deal with a multiplicity of frequencies and require the same reliability as that of the central office, it is very expensive. In addition, expansion of the network and rearrangement of the subscriber terminals are not made easily and the flexibility of the network is lacking.